So, I watched both OVA's and now can write a review on it.
In short: the first is about Chihiro and how he had found a book about resurrection; the second is about Aria and her strange dream (I can't write what was this dream about because it will be a great spoiler for you; for same reason I'm not writing who is Chihiro&Aria).
First OVA is like a prequel for the TV series. If you didn't watched "Sankarea" yet, I strongly recommend you to start with this OVA -- you'll free from several question and won't say "WTF?!" after the first TV ep. 10/10.
Second is so
strange and weakly connected to the main story. That is why I recommend you watch this after the TV series, but not the right after! 6/10.
Both OVA's are well-animated and have cute scenes. Quint essence of the TV series...
Nevertheless, I can't say that it's a masterpiece.
Just like I said about art. Also 9/10.
Well, well. All characters, except Aria in her dream, are shown true. True, but not great.
7/10, also due to better first and worse second OVA.
8/10. Very, very good additional episodes of "Sankarea". I like it.
If you haven't watched Sankarea yet, definitely watch the first OVA, (Episode 00), before you do. Being the prequel, it'll answer some questions you have.
Episode 00 is about 17 months earlier. Chihiro is telling a story, about memories, mostly in his perspective but also some is in Rea's. You see some more about Rea's odd relationship with her father and her mother. Chihiro goes on a holiday with his family and Ranko tags along. So you learn more about their history, too.
The second OVA, Episode 14, is best to watch after the series, as that's it's proper place. A weird girl appears under the
temple and strange things start happening. Chihiro shows he's smarter than he looks, and it all comes together in the end.